Symbol indicating reply to mail missing (Office 2000 SR1)

Starting a few months ago, I noticed that the little arrow indicating that a mail was replied to is only being shown about half the time. It is frustrating that this is not working 100% of the time since I receive a lot of e-mails and it is difficult to keep track of which mail I have replied to when the arrow doesn't automatically appear reminding me that I have replied to an e-mail.